This invention relates to the general field of digital computers and more particularly to apparatus for generating signals for controlling the exact time of data acquisition by a digital computer in a real time data acquistion system.
While digital computers have long been used for the purpose of processing recorded data, their use in the field of real time data acquisition is relatively new. In a real time data acquisition system, a digital computer is wired directly to a remote piece of laboratory test equipment, such as a voltmeter or pressure detector. In this way, the laboratory equipment output is read directly into the computer memory without the time delay and possible error caused by human intervention.
A source of errors has been recognized in such real time data acquisition systems. Since relatively long conducting lines must be connected between the laboratory equipment and the computer, electrical noise from a variety of sources, primarily associated with powerlines, is picked up by the signal-conducting lines. Powerline noise which is picked up in this way tends to obscure the signal, or, in other words, to reduce the signal-to-noise ratio. The accuracy of the system is thus degraded, and the information recorded by the computer may be unreliable. It has been recognized that the noise levels on such signal lines are at a minimum at time periods corresponding to the zero crossings of the AC powerlines and harmonics of the AC powerline frequency. Thus, the signal-to-noise ratio of acquired information on such lines is a maximum when the data is acquired precisely at the zero crossing time occurrences.
Based on the above information, attempts have been made to synchronize computer data acquisition with the zero crossings of the powerline and harmonics of the powerline frequency. In general, these attempts have involved connecting analog comparators to the line frequency signal to give zero crossing information for the line frequency itself. Zero crossings of harmonics of the line frequency were obtained by using analog resonant circuits having resonant frequencies at the desired harmonics, driving the resonant circuits with the line frequency signal, and using analog comparators to detect the zero crossings of the resonant frequency signals. Several problems have arisen in practice with such circuits. It is difficult for the analog comparator to accurately identify a zero crossing where the input signal is a sine wave, such as the line signal itself, or the output of a resonant circuit. The results of this type of inaccuracy cause both jitter and an average error in the timing of the zero crossing indication. Such an error is magnified in the case of the higher harmonics due to further errors introduced by the resonant circuits. The analog resonant circuits used at these low frequencies, below 1000 Hz, are unstable due to component drift, particularly in capacitors. Such component drift can be compensated for by use of adjustable elements but need for continual readjustment is undesirable.
There has been a need, therefore, for a highly accurate, stable, and reliable apparatus for generating indications of the zero crossings of powerline signal and harmonics thereof.